Jubilant Mascot is a uncommon Creature — Homunculus card from The List (PLST), collector number BBD-28. Current market price: $0.15, TCGplayer market prices via Scryfall.
At the beginning of combat on your turn, you may pay {3}{W}. If you do, support 2. (Put a +1/+1 counter on each of up to two other target creatures.)